Among petroleum products, for example, lubricant oils, gas oils, jet fuels, and the like are products in which cold flow property is considered to be important. For this reason, it is desirable that base oils used for these products be such that waxy components such as normal paraffins or slightly branched isoparaffins, which are responsible for being improved the cold flow property, have been completely or partially removed, or converted to components other than waxy components. Hydrocarbons obtained by Fischer-Tropsch synthesis (hereinafter sometimes referred to as “FT synthesis”) have attracted attention as feedstocks for producing lubricant oils or fuels, because they do not contain substances of concern such as sulfur compounds; however, these hydrocarbons also contain many waxy components.
An example of a known dewaxing technique for removing waxy components from hydrocarbon oils is a method wherein waxy components are extracted using a solvent such as liquefied propane or MEK. However, this method has problems in that the operating costs are high, the types of usable feedstocks are limited, and the product yield is limited by the type of feedstock.
On the other hand, an example of a known dewaxing technique for converting waxy components in a hydrocarbon oil to non-waxy components is catalytic dewaxing in which the hydrocarbon oil is contacted, in the presence of hydrogen, over a bifunctional hydroisomerization catalyst capable of hydrogenation-dehydrogenation and isomerization, thereby isomerizing normal paraffins in the hydrocarbon oil to isoparaffins. Further, examples of known hydroisomerization catalysts used for catalytic dewaxing include catalysts containing solid acids, represented by for example, zeolites, and metals belonging to Groups 8 to 10 or Group 6 of the periodic table.
While catalytic dewaxing is an effective method for improving the cold flow property of hydrocarbon oils, it is necessary to sufficiently increase the normal paraffin conversion in order to obtain a fraction that is suitable for a lubricant base oil. However, because the hydroisomerization catalysts used in the catalytic dewaxing are capable of both isomerization and hydrocarbon cracking, the cracking reaction of the hydrocarbon oil into lighter products also proceeds as the normal paraffin conversion increases, reducing to yield a desired fraction. Particularly, when producing a high-quality lubricant base oil in which a high viscosity index and low pour point are required, it is necessary to increase the conversion to the extent that the normal paraffin is not substantially contained and is hence very difficult to economically obtain a desired fraction by the catalytic dewaxing of a hydrocarbon oil. For this reason, synthetic base oils such as polyalphaolefins have been frequently used in this field.
Under such circumstances, there is a need for a hydroisomerization catalyst having high isomerization activity while suppressing undesirable cracking, i.e., having excellent isomerization selectivity, for the purpose of obtaining a desired isoparaffin fraction in good yield from a hydrocarbon oil containing waxy components in the field of producing lubricant base oils and fuel base oils, particularly lubricant base oils.
Attempts to improve the isomerization selectivity of hydroisomerization catalysts used in catalytic dewaxing have been made in the past. For example, Patent Literatures 1 to 5 listed below discloses a method for producing a dewaxed lubricant oil, wherein a straight-chain or slightly branched hydrocarbon feedstock having 10 or more carbon atoms is contacted under isomerization conditions with a catalyst comprising a zeolite, such as ZSM-22, ZSM-23, or ZSM-48, having one-dimensional pores of an intermediate size and containing a metal of Groups 8 to 10 or the like of the periodic table, and having a crystallite size of no more than about 0.5μ.
It is noted that a zeolite that constitutes a hydroisomerization catalyst is typically produced by hydrothermal synthesis in the presence of an organic compound which is called an organic template having an amino group, ammonium group, or the like, in order to construct a predetermined porous structure. The synthesized zeolite is then calcined in an atmosphere containing molecular oxygen at a temperature of, for example, about 550° C. or more, to thereby remove the organic template contained therein, as described in, for example, the final paragraph of the “2.1. Materials” section on page 453 of the Non Patent Literature 1 listed below. Next, the calcined zeolite is typically ion-exchanged into an ammonium form in an aqueous solution containing ammonium ions, as described in, for example, the “2.3. Catalytic experiments” section on page 453 of the Non-Patent Literature 1. A metal components of Group 8 to 10 or the like of the periodic table is further supported on the ion-exchanged zeolite. The zeolite on which the metal component is supported is then subjected to steps such as drying, and optionally extruded, and then loaded in a reactor; the zeolite is typically calcined in an atmosphere containing molecular oxygen at a temperature of about 400° C., and is further subjected to reduction treatment with, for example, hydrogen, at about the same temperature; consequently, the zeolite is provided with catalytic activity as a bifunctional catalyst.
On the other hand, catalysts for commercial use is typically used in the form of a extruded product for the purpose of improving the handleability, reducing the pressure loss of a reaction fluid in the catalyst bed, and the like. However, zeolite powders have lower cracking activity and the mechanical strength of a catalyst prepared from a extruded product obtained by extruding only such a powder is small, hence making it difficult to apply for practical use. Consequently, catalysts in which zeolite is used are typically used in the form of a extruded product prepared by extruding a composition of zeolite powders to which an inorganic oxide powder called a binder is added.